Recently, Ethernet-based services have been more frequently utilized owing to the explosive spread of communications over the Internet or the like. The provision of services in the widest possible range and at the lowest possible cost may be important for dealers of communication services. In general, the dealers of communication services construct their systems by utilizing existing network equipment as effectively as possible. Under the above mentioned circumstances, data is transmitted by utilizing the transmission network of highly reliable Synchronous Optical Networking/Synchronous Digital Hierarchy (SONET/SDH) which has been constructed so far so as to cover a wider area.
A system of transmitting Ethernet-based packet data over the SONET/SDH transmission network is widely used as the Ethernet over SONET/SDH system and the VCAT (Virtual Concatenation) is generally used as a mapping system thereof (by way of example, a technique disclosed in Japanese Laid-open Patent Publication No. 2002-232380 is given). In addition, in the SONET/SDH system, a redundancy configuration including a current-use system (a work system) and a spar system (a protect system) is adopted in order to avoid interruption of a service caused by occurrence of a fault.
FIG. 1 is a diagram illustrating an example of a configuration of an existing device that transmits packet data from a client over the SONET/SDH transmission network. In the device illustrated in FIG. 1, UPSR (Uni-directional Path Switched Ring) is thought to be used as a protection function. Incidentally, in FIG. 1, although only the configurations of a source transmission device 1000 and a sink transmission device 2000 with SONET/SDH transmission networks interposed between them are illustrated, in a typical transmission device, both the source and sink configurations are included in one transmission device.
In the example illustrated in FIG. 1, in the source transmission device 1000, a client data receiving unit 111 receives packet data from a client. An SDH mapping unit 112 maps the received packet data on a concatenation group and divides the data into “n” paths. Then, a protection switch unit 113 distributes the data of the respective paths to two systems of the work system and the protect system using a distributor unit. The data so distributed is multiplexed using an interface unit 114 and is transmitted to a SONET/SDH transmission network of the work system and a SONET/SDH transmission network of the protect system.
On the other hand, in the sink transmission device 2000, an interface unit 121 receives the data from the SONET/SDH transmission network of the work system and the data from the SONET/SDH transmission network of the protect system and restores the multiplexed data respectively sent over the network of the work system and the network of the protect system into the data which is divided into the respective paths. Then, a protection switch unit 122 selects a path of the work system or the protect system using a selector unit. Next, an SDH demapping unit 123 stores the data on the respective paths into a differential delay memory unit 124, by taking correspondence of the data with the concatenation group into consideration. Then, a client data sending unit 125 regenerates the packet data by reading the data out of the differential delay memory unit 124 at the speed of the client on the sink side and sends the data to the client.
Incidentally, the differential delay memory unit 124 is provided for reasons as follows. That is, in the VCAT, unlike general continuous concatenation, main signal data is transmitted by dividing it to respective paths which configure a concatenation group and each path is handled as a discrete path on the SONET/SDH transmission network, so that it may not be guaranteed that the respective paths are transmitted along the same route and hence it may sometimes occur that a difference (differential delay) is generated in transmission delay time among the respective paths because the respective paths are transmitted along different routes. The differential delay memory unit 124 performs buffering (to temporarily store data in a buffer) on the data of the respective paths which configure the same concatenation group to wait for data from the path which comes late, thereby reassembling the packet data. The differential delay which may be absorbed is determined depending on the size of the differential delay memory unit 124 and a network may be limited by the size of the differential delay memory unit 124. In the case that the differential delay of the amount that the differential delay memory unit 124 may leave unabsorbed has generated among the respective paths, the signal in the concatenation group concerned may not be reassembled and hence the signal may not traffic.